The present invention relates to a robot machine and particularly relates to an origin point adjustment method thereof.
The instructing process of a prior art teaching playback type robot is performed while the robot being actually operated. Because of such an instructing method as above, a lot of teaching playback type robots have been widely used in the industry and the market thereof is still expanding. The operating accuracy of a teaching playback type robot is positional accuracy in reproducibility of a teaching point. Therefore, the individuality of each respective robot and absolute positional accuracy have not been considered important. The foregoing tendency is still prevailing.
In recent years, however, a demand for a reduction in teaching steps of teaching playback type robots has been growing and there are great expectations upon an off-line teaching process that allows the teaching process and performance simulation of robots to be taken place on a display with the use of a computer. By living up to the expectations, many off-line teaching systems have been introduced to the market and actually in use.
However, the positional difference existing between the structural model of a robot defined inside a computer and the actual robot in use becomes a serious cause of degrading the teaching accuracy involved. Therefore, it is necessary at present for a teaching program prepared by the use of an off-line teaching system to be corrected at worksite and actually many corrective teaching instructions are provided.
In addition to the positional accuracy, a high degree of accuracy has been required of an operating path when a CP(continuous path) operation is taking place.
In order to satisfy the foregoing requirement, considerable efforts have been put into enhancing the computing speed and accuracy of a robot controller and achieving a higher degree of accuracy in machining the respective mechanical parts that constitute robot arms. However, in the same way as in the case of off-line teaching, the positional difference existing between the structural model of a robot defined inside a robot controller and the actual robot has a great effect on the accuracy of an operating path. Therefore, market requirements with respect to operating path accuracy at the time of CP operation are not allowed to be satisfied sufficiently.
The positional difference between the structural model of a robot and an actual robot causes adverse effects on individuality, absolute positional accuracy and operating path accuracy of the robot.
The foregoing positional difference is caused by machining accuracy of arm length, twisting between axes and the like, assembly accuracy and shifting in position of the original point of rotational joint axis. Above all, the shifting in position of the original point of rotational joint axis has the greatest influence to cause the positional difference.
As the method of calibration is considered a method for applying a correction to a structural model by taking measurements of a positional difference between the structural model of a robot and an actual robot. This calibration method is being studied for calibration purposes of robots. A few different methods are proposed as the calibration method. However, since the measurement of the positional difference needs to be performed with an extremely high degree of accuracy, the method of measurement itself is difficult and requires very expensive measurement instruments. As a result, the method of applying calibration to a structural model has not so far prevailed in the industry. Therefore, the inventor of the present invention proposed a method of using a tilt angle sensor in the Japanese Patent Application Unexamined Publication No. H05-318351 as the method for facilitating automated measurement at a relatively low cost.
Next, a brief description is given to a prior art method with reference to FIG. 1, FIG. 7 and FIG. 8. Particularly, FIG. 1 is used for guidance in order to make the description simple.
FIG. 1 is a perspective view of an example of the robots that are targeted for an origin point adjustment. Robot 1 in FIG. 1 comprises six rotational joint axes of first joint axis J1 to six joint axis J6. As FIG. 1 shows, respective joint axes J1 to J6 are allowed to be moved in rotational directions a to f by controller 2 that controls the motion of the robot.
FIG. 7 is a conceptual illustration of an original point adjustment method as employed in a prior art robot and shows in a schematic form third joint axis J3, fourth joint axis J4 and tilt angle sensor 3 mounted on the tip of robot arm for the robot as shown in FIG. 1, for example. As FIG. 7 shows, fourth joint axis J4 is tilting by angle xcex1 from direction G of gravity due to the rotation of third joint axis J3 that makes the center axis of rotation for fourth joint axis J4. Tilt angle sensor 3 is mounted on the tip of robot arm via fifth joint axis J5 and six joint axis J6. (Refer to FIG. 1.)
In the foregoing, the sensor mounting surface serving as a reference surface of detection for tilt angle sensor 3 is assumed to make a tilting angle of xcex3 (not shown in FIG. 1) against fourth joint axis J4.
In addition, an encoder (not shown in FIG. 1) acting as means for measuring a rotational angle of joint axis is attached to each respective rotational joint axis.
With a robot that is structured as described in above, while fourth joint axis J4 being rotated by controller 2 (refer to FIG. 1), tilt angle xcex3 and rotational angle "THgr" of joint axis are measured at predetermined rotational angle positions by tilt angle sensor 3 and the encoder, respectively.
FIG. 8 shows the relationship between rotational angle "THgr" of fourth joint axis J4 (the same "THgr" as shown in FIG. 7) and tilt angleY measured according to the original point adjustment method as described in above. In FIG. 8, the maximum value of tilt angleY is Ymax, the minimum value is Ymin and, when tilt angleY equals to the mean value of the maximum and minimum values thereof, a rotational angle of fourth joint axis J4 is defined as "THgr"zero.
As is evident from FIG. 7 and FIG. 8, tilt angle xcex1 of fourth joint axis J4 against direction G of gravity can be derived by calculation in such a way as dividing the difference between the maximum valueYmax and minimum valueYmin of the measured tilt angleY by two. Further, the direction of tilt is allowed to be measured with reference to "THgr"zero.
Thus, the original point position of third joint axis J3 can be readily adjusted by the use of a value of tilt angle xcex1 of the rotational center of fourth joint axis J4 measured and derived by calculation as described in above. The foregoing adjustment method can be applied to other axes equally well.
As described in above, according to the prior art original point adjustment method, tilt angle xcex1 of a rotational joint axis against direction G of gravity can be derived from the difference between the maximum and minimum values of tilt angleY. On the other hand, since tilt angle xcex1 is not affected by mounting angle xcex3 of tilt angle sensor 3, an adjustment with a high degree of accuracy is made possible without being affected by machining accuracy and the like of the reference surface for mounting the sensor.
In addition, as described in above, mounting angle xcex3 of tilt angle sensor 3 does not have any influence with respect to measurement, thereby allowing the original point position of each respective joint axis to be adjusted in succession as the attitude of robot 1 is being changed as appropriate. Therefore, once a single sensor is attached to the tip of robot arm, adjustments of a plurality of joint axes are made possible by having a rotational angle of each respective joint axis of robot positioned appropriately, thereby allowing even automated adjustment steps to be realized without difficulty.
However, the prior art method has had the following two flaws. Firstly, tilt angle Y and rotational angle "THgr" are measured continuously and it is needed to find out precisely at what positions of rotational angle tilt angle Y falls on maximum value Ymax and minimum value Ymin. In order to measure the maximum value Ymax and minimum value Ymin of tilt angle Y, it is necessary for rotational angle "THgr" to be rotated by more than 180xc2x0 at least. However, when there is not much extra room for the place where a robot is installed, rotational angle "THgr" is sometimes not allowed to be rotated by more than 180xc2x0, thus bringing about the situation where carrying out a measurement itself is difficult.
Secondly, the prior art method has another flaw in that the original point adjustment of an xe2x80x9cnxe2x80x9dth joint axis is performed by the use of a tilt angle of rotational center axis of an xe2x80x9cn+1xe2x80x9dth joint axis. As observed with second joint axis J2 of robot in FIG. 1, for example, when parallelism with a rotational joint axis adjoining thereto, i.e., third joint axis J3 is mutually maintained, the prior art method does not allow the adjustment of second joint axis J2 to be performed, thereby requiring the adoption of a different method to carry out the adjustment.
The present invention deals with the foregoing problems and provides a method of original point adjustment whereby a high accuracy adjustment can be performed even if the room where a robot is allowed to move is limited in space and also an adjustment can be carried out even if joint axes are located in such a way as adjoining rotational joint axes are parallel with one another.
In order to solve the foregoing problems, the present invention proposes with respect to rotational joint axes of a robot machine having a plurality of rotational joint axes, in which adjoining rotational joint axes are not parallel with one another, a method of original point adjustment comprising the steps of:
measuring a rotational angle of rotational joint axis and a tilt angle of robot arm against the direction of gravity at three points or more of rotational angle position at least;
deriving by computation regression factors xcex1 and xcex2 based on a regression function of Y=xcex1 SIN("THgr"+xcex2)+xcex3 formed of thus obtained rotational angle information "THgr"i and tilt angle information Yi ; and
performing an original point adjustment based on the obtained tilt angle and tilt direction of rotational joint axis.